1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method and system for improving the performance of handovers between different mobile communication systems.
2. Description of Related Art
While performing a handover of a mobile terminal from a Global System for Mobile Communications (GSM) network to a Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN), the UTRAN Radio Resource Control (RRC) xe2x80x9cHANDOVER TO UTRAN COMMANDxe2x80x9d message is to be transferred to the mobile terminal via the GSM network. In order to achieve adequate performance, it is essential that the UTRAN RRC message be transferred to the mobile terminal within a non-segmented GSM radio air interface message. However, in the GSM, the air interface signalling uses a window size of 1. As such, a GSM Base Station Controller (BSC) will not transfer a second message segment prior to receiving an acknowledgment message for a first message segment. Consequently, if the uplink connection is of relatively poor quality when a handover is initiated, the BSC may never receive an acknowledgment message for the first message segment and thus will be unable to convey the second message segment to the mobile terminal. Nevertheless, in the case where the UTRAN RRC message fits into a non-segmented GSM air interface message, the handover can be performed regardless of the quality of the GSM uplink connection.
In order to avoid performance problems, the size of the UTRAN RRC HANDOVER TO UTRAN COMMAND message should be less than 18 octets or 144 bits. This result can be achieved by using predefined parameter settings and transferring information after the handover procedure has been completed. However, this approach of using predefined parameter settings reduces flexibility, and the handover performance can degrade if the activation of certain non-essential functions (e.g., multiple radio links, SSDT, etc.) is deferred until the handover procedure has been completed.
Certain network operators consider the immediate activation of the functions more important than avoiding the problems related to the use of multiple GSM message segments. In order to support these particular operators, it is possible to make allowances for optional parameters. However, the use of optional parameters results in an increase of the message length even when such a parameter is not included in the message. In other words, the use of optional parameters affects the minimum message size. Currently, given the present state of the art, it is not possible to allow the use of all desired optional parameters, because that use would imply that a message would no longer fit within a single segment. Accordingly, there is a need for a solution whereby a handover message will have room to include additional (e.g., optional) parameters. As described in detail below, the present invention successfully resolves the above-described problems and other related problems.
In accordance with a preferred embodiment of the present invention, the sizes of certain (e.g., S-RNTI and scrambling code number) Information Elements (IEs) in the. UTRAN RRC HANDOVER TO UTRAN COMMAND message can be reduced, by reserving a subrange of the IE parameter values for exclusive use by the User Equipment (UE) which is performing an inter-Radio Access Technology (RAT) handover to a UTRAN. For example, the lowest values of scrambling code number or Serving-Radio Network Temporary Identity (S-RNTI) number, which is part of a Universal-RNTI (U-RNTI) number, can be reserved for initial assignment to a UE performing a handover to the UTRAN. Upon completion of the handover procedure, the UTRAN can assign another scrambling code number or S-RNTI-number to that UEE. Consequently, by significantly reducing the size of the IEs used in the HANDOVER TO UTRAN COMMAND message, additional (e.g., optional) parameters may also be used. As such, the space that becomes available in the HANDOVER TO UTRAN COMMAND message may also be used to create options for avoiding the use of predefined radio configurations, which results in increased flexibility.
An important technical advantage of the present invention is that the quality of handover procedures to UTRANs from other system networks is significantly improved.
Another important technical advantage of the present invention is that an improved handover command message can be transmitted from a system network to a UE within a non-segmented air interface message.
Still another important technical advantage of the present invention is that optional parameters can be included in a handover command message from a system to a UE, which increases the performance of handovers to a UTRAN from other system networks.
Still another important technical advantage of the present invention is that the size of the information elements in a handover command message can be reduced so that a single, non-segmented message can be transferred in its entirety across a (GSM) radio air interface.